Cyclone furnaces or burners are often the firing units of boilers. A cyclone furnace is formed with a horizontally disposed tube, or furnace box, into which the fuel is introduced at one end, and the combustion gases are expelled at the opposite end. These combustion gases rise, transferring their heat to water or steam flowing in the boiler tubes above to convert the water to steam or to superheat the steam. From the steam, power such as electricity is produced.
In a cyclone furnace the air required to combust the fuel is added in three modes. For coal fired cyclone furnaces, crushed coal is conveyed into the furnace with the first or primary air stream, the first mode of adding air. The primary air stream generally comprises about 15 to about 20 percent of the total required air. A major proportion of the required air, about 65 to about 80 percent, is introduced into the furnace in a secondary air stream, a high speed stream added tangentially to the furnace tube or box at its circumference. About 5 percent of the required air is introduced as a tertiary air stream which fine tunes the total amount of air being added to the furnace.
The tangentially added secondary air stream creates a rotating air movement within the furnace box wherein the air whirls inward toward a center of minimum pressure, resembling a horizontal cyclone. The crushed coal being fed then moves through the center of this whirling air formation, from the box end where it was added with the primary air stream, to a flame where it is combusted. The hot gases of combustion, which are mainly carbon dioxide and water vapor, are emitted at the far end past the flame.
The nongaseous combustion products of coal are coal ash or slag. Such slag is generally composed of compounds of silicon, aluminum, iron, calcium, and possibly some titanium, phosphorus, and the alkali metals. The chemical composition may vary over a wide range, particularly with respect to the compounds of silicon, aluminum, and iron, depending upon the source of the coal. Even coal derived from different seams in the same geographic region can have slags or coal ash of significantly varied compositions.
All coal ashes, when heated to a sufficiently high temperature, will form a liquid slag whose viscosity varies inversely with temperature. It is generally believed in the art that chemical reactions occur in the ash during liquid slag formation, and that the viscosity patterns of the slag so formed are dependent on both the ultimate composition of the slag and the state of oxidation of the iron therein.
When coal is combusted in a cyclone furnace, about 10-15 percent of the combustion product will be coal ash. This percentage can vary from about 5 to about 35 percent for some unusual coals. A substantial portion of this is slag driven by the centrifugal force created by the secondary air stream to the furnace wall. For some cyclone furnace, about 85 percent of the slag formed will go to the walls, the remainder leaving the furnace with the combustion gases, and fly ash.
Wet bottom cylcone furnaces are designed for removal of the slag in its molten state, and have drain holes at the bottom of the box. For a given slag, however, there is a temperature at or below which the slag will not effectively flow down the walls of the furnace to and through the slag drain holes under the low shear forces of gravitation. This temperature is generally called the temperature of critical viscosity. Further, at an even lower temperature, the slag freezes to a solid. Related to these temperature is a number of temperatures discussed below, which can be easily determined in a laboratory, and it is recognized in the art that a change in the laboratory determined temperatures is indicative of a similar change in the freezing temperature and temperature of critical viscosity of a given slag.
Thus the viscosity pattern of a given slag is temperature dependent, and dependent on the ultimate composition of the slag which in turn depends on the composition of the coal being combusted and probably to an extent on the combustion conditions.
Thus a wet bottom cylcone furnace may have been designed for effective removal of slag having a given temperature necessary for effective flow and that furnace under operating conditions creates at least such minimum temperature environment at its walls. But due to changes or fluctuations in coal composition, or the need to burn less expensive coal, the wall temperature environment is not sufficient for the coal actually being combusted. The slag does not flow effectively. Even the drain holes become clogged.
This slagging problem is further complicated when coal particles, generally those of larger than desired size, escape combustion in the flame and are driven to the furnace walls with the slag, forming a matrix with the molten slag and thus disrupting the slag flow. It is believed that the presence of such coal particles at the furnace walls will significantly reduce or stop effective slag flow even though the temperature environment at the walls would otherwise be sufficient for the slag being produced.
Moreover, when slag flow slows down or stops, that slag will be covered with layers of more slag, forming not only a thicker build-up, but reducing the temperature environment of the slag below the outermost layer. Heat transfer through slag is low, slag being considered generally an insulating material. Thus a condition that began as a slowing up of the slag flow may easily become one of frozen slag build-up due to the significant drop of the temperature gradient from the outermost slag layer through to the furnace walls.
Thus the disruption of slag flow by the presence of uncombusted coal particles at the furnace walls, even when the coal being combusted is compatible with the design of the particular wet bottom furnace, can lead to a serious slag build-up problem.
When slag builds up on furnace walls, it distorts the burner's flame configuration, and the greater the build up, the greater the distortion until the furnace is required to be shut down. Utility boilers are generally fired by a plurality of cylcone furnaces and require a given minimum of these for operation. If a sufficient number of furnaces are shut down, the boiler itself must be shut down. Thus furnace slagging problems not only create expensive maintenance costs in the cleaning of slagged over furnaces, but the lead to the expense of lost production time and the expense of purchasing the product, such as electricity, from other producers to meet the needs normally served.
Chemical slag modifiers are well known in the art. For wet bottom furnaces, suitable slag viscosity modifiers reduce the fusion point of slag to achieve the necessary slag viscosity in the temperature environment present at the furnace walls. Such slag viscosity modifiers, for example, include without limitation sodium sulfate, sodium carbonate, borate salts of ammonium, lithium, magnesium, potassium and sodium, and other alkaline salts, and minerals such as dolomite, colemanite, limestone, and ulexite.
To reduce the number of coal particles that escape the flame uncombusted, it is well known in the art to add a combustion catalyst or adjuvant, such as salts of copper, iron, cobalt, managanese, and the like.
Further it has been the general practice to feed such slag modifiers and combustion catalysts to the furnace as part of the coal feed, as intimate mixtures with the pulverized coal. Such additives are generally introduced into the furnace on a continuous basis at levels generally within the range of from about 0.1 up to even 100 pounds per ton of coal being fed to the furnace.
A portion of slag viscosity modifiers added with the coal feed is presumed to become intimately mixed with the slag as it is formed in the flame area, and be driven to the furnace walls with the slag. Combustion adjuvants when added to the coal feed presumably are present in the flame to promote combustion of the larger particles in the coal feed. A significant portion of both, however, becomes entrained in the combustion gases and is removed from the furnace box, never reaching the furnace walls. The additives, when added to the coal feed, thus do little to aleviate slag build up that is caused by the intermingling of uncombusted coal particles with the slag, other than to reduce the number of such particles, but in practice the additives do not reduce the coal particles to zero.
These additives add significantly to the cost of producing electricity or other power when used at typical levels. Further, at desired use levels, some of the additives have deleterious effects, such as the sodium compounds which create corrosion problems, limiting the use of sodium compounds although they are well recognized as extremely effective fusion point modifiers. As mentioned above, even if the slag is properly modified by the viscosity modifiers added with the coal feed, if the combustion adjuvants do not reach the furnace walls, viscosity problems due to the presence of uncombusted coal particles will result.